Rotor assemblies are used in electrical machines, such as in automotive vehicle alternators, and include selectively rotatable magnetic pole members which selectively and operably cause electricity to be generated. One common type of alternator, known as a "Lundell" or "claw-pole" type alternator, generally includes a rotor having two opposed "claw-pole" halves or "pole pieces" which are operatively secured to a selectively movable rotor shaft. Each of these pole pieces includes several pole fingers. As the pole pieces are operatively assembled upon the rotor, the respective pole fingers selectively and cooperatively "intermesh", thereby forming a rotor assembly having north and south magnetic poles. In order to increase power density, efficiency, and to reduce rotating inertia, permanent magnets are typically inserted into the rotor assembly and are oftentimes secured between the operatively assembled pole pieces.
One design or method of securing the permanent magnets within the rotor assembly is by "press fitting" a magnet under each of the "tips" or the ends of the pole fingers. One drawback associated with this arrangement is that the magnets will often become loose and/or "fall out" of the assembly due to the centrifugal force generated by the very high rotational speeds of the rotor assembly. Another drawback associated with this arrangement is that the force, stress and/or pressure required to be imparted upon the magnets, as they are press-fitted into their respective operative position within the rotor assembly, causes the relatively brittle magnets to fracture and/or crack. Yet another drawback associated with this prior arrangement is that the magnet surfaces must be formed or created within very "tight" tolerance limits in order to allow the magnets to be relatively smooth, flat and to have particular structural dimensions which allow them to be securely fixed underneath the pole fingers. These "close" tolerances require relatively expensive and time consuming machining processes.
Other prior methods of retaining magnets within the rotor assembly utilize additional components, such as rings, stamped cups and over-molded magnets to "fix" the magnets in the desired positions. These methods increase the overall production cost of the rotor assembly while undesirably increasing the rotor's structural complexity, thereby increasing the probability of component failure. Furthermore, the use of these additional "magnet securing components" creates undesirable gaps between the pole pieces and the magnets, thereby decreasing the electrical output and efficiency of the assembly. Finally, these prior methods do not substantially allow for direct and desirable contact between the respective magnet surfaces and the rotor pole pieces and prevents and/or reduces the amount by which the magnets may be cooled during operation of the rotor assembly.
There is therefore a need to provide a rotor assembly which overcomes the various and previously delineated drawbacks of the various prior assemblies; which includes several selectively and fixedly secured permanent magnets; which does not require a relatively high amount of force, stress or pressure to be imparted upon the permanent magnets as they are secured within the rotor assembly; which does not require the magnets to undergo expensive machining processes; and which does not require additional "magnet securing components" to be included within the rotor assembly.